This invention relates in general to electromagnetic assemblies, and in particular, to armature-type, magnetic holding assemblies.
In conjunction with mobile hydraulic equipment it is often desirable to provide such equipment with a feature for returning one or more of the elements operated by the equipment to a predetermined position. For example, on a frontend loader, it is desirable to have the bucket automatically returned to a level position and thereby be ready for the next load. Such automatic return features can be provided by biasing or detenting the movable valve element, such as the spool valve of a hydraulic control valve, to a predetermined position by suitable means. There are various means available to accomplish such detenting including mechanical means as well as electromechanical means.
A typical electromechanical means is a holding type electromagnet which acts on an armature plate that is mechanically coupled to one end of the spool by a stem. When the electromagnet is energized, the armature plate is held against the core of the electromagnet thereby completing the magnetic flux path and thus holding the spool member in the desired position against the normal opposing forces of the spool's centering springs. Such electromagnets typically are flat faced, armature-type magnetic holding coils assembled with one end of the core open and facing the armature plate. When the electrical energy to the coil is removed, the magnetic flux collapses and the spool's centering springs separate the armature plate from the core and return to the spool to its center or neutral position.
The electromagnetic assembly typically comprises a magnetic core having an outer cylindrical portion, an inner cylindrical portion, and a cavity between the two portions adapted to receive a bobbin that has been prewound with a coil of wire. The bobbin has to be retained by some means in the core cavity in order for the electromagnet to operate properly. However, there is little space available for any retaining means in order to achieve this purpose. This is so because the core area necessary to carry the magnetic flux is limited and therefore not available for any such retaining means.
Until now, one method used to retain the coil in place has been to glue the bobbin to the closed inner wall of the core cavity. Such a method is effective, but has several drawbacks. For one, it is time consuming inasmuch as the assembled unit will require drying time for the sealant to set. For another, care must be taken that the coil stays in its position until the sealant has dried and that all sealant is cleaned from the open face of the core and bobbin so as not to interfere with the armature plate. If the coil becomes damaged, then the entire assembly, including the relatively expensive core portion, has to be completely scrapped since the bobbin will remain permanently fixed to the core by the sealant.
Accordingly, it would be desirable to have another means for removably fixing the bobbin and coil in the core cavity.
In conjunction with other types of electromagnetic devices, there have been described a number of methods for holding a bobbin to a core. For example, U.S. Pat. No. 3,428,929 relates generally to a coil structure for tuned circuits in radio and television amplifiers. In that patent, there is described a method for locking a bobbin onto a core having a single cylindrical portion. The bobbin is locked onto the core portion by the windings of the coil which abut against the core through windows provided in the bobbin. An arcuate projection of the inner cylindrical portion of the bobbin is received in an elongated longitudinal depression in the surface of the core to enable the bobbin to be slid into its proper position.
In another U.S. Pat. No. 3,238,485, there is shown a bobbin, having a number of small, molded projections on its inner surface of a hollow tubular portion. The projections, resembling pin-type projections, can be forced into a corresponding shallow depression or groove in a cylindrical core in order to hold the core in place inside the bobbin.
In U.S. Pat. No. 4,048,606, there is described an inductive device which is said to reduce creepage by providing a bobbin with flanges having thinned-down edges for bending over a portion of the coil when the bobbin is inserted into a core. No mention is made of how the bobbin is secured to the core.
Still other examples of bobbin and core assemblies may be found in U.S. Pat. Nos. 4,114,056; 3,958,328; 3,843,946; and 3,314,032.